The invention relates to a magnetic resonance imaging method comprising the steps of
acquisition of sub-sampled magnetic resonance signals with a system of receiver antennae
the system of receiver antennae having a spatial sensitivity profile
reconstruction of a magnetic resonance image on the basis of
the sub-sampled magnetic resonance signals
the spatial sensitivity profile and
a priori image information
optimization of the reconstruction with respect to a pre-selected aspect of distribution of sampled data included in the sub-sampled magnetic resonance signals over the reconstructed magnetic resonance image.
Such a magnetic resonance imaging method is known from the paper xe2x80x98SENSE: Sensitivity encoding for fast MR-imagingxe2x80x99 in Magnetic Resonance in Medicine 42 (1999) 952.
This magnetic resonance imaging method is commonly known as the xe2x80x98SENSE-methodxe2x80x99 in the field of MR-imaging. In the known magnetic resonance imaging method the number of phase-encoding steps is reduced so as to reduce the time required for the acquisition of magnetic resonance signals. Consequently, the acquired magnetic resonance signals are built-up as superpositions of contributions from several positions in the scanned volume. The spatial sensitivity profile of the system of receiver antennae is employed to decompose the superposed contributions into signal amplitudes relating to separate positions in the scanned volume. These signal amplitudes represent the brightness values in the magnetic resonance image at full sampling. In other words, part of the spatial encoding of the magnetic resonance signals is performed on the basis of the spatial sensitivity profile. The decomposition of the sub-sampled magnetic resonance signals into signal amplitudes involves a reconstruction matrix which relates the signal amplitudes to the sub-sampled magnetic resonance signals.
The cited reference mentions that different optimizations may be employed in the reconstruction. The so-called strong reconstruction derives the reconstruction matrix from a close approximation to a pre-selected spatial encoding. In particular the strong reconstruction is carried out as a least-squares approximation to a pre-selected set of voxel functions which represent the pre-selected spatial encoding. The so-called weak reconstruction derives the reconstruction matrix from a close approximation to a pre-selected noise distribution in the reconstructed magnetic resonance image.
The known SENSE-method employs a priori knowledge in the reconstruction in that for positions which are outside the object to be examined the pixel-value is set to zero in the reconstructed magnetic resonance image. However, this a priori information is difficult to acquire and the cited reference hardly provides any effective measures to obtain this a priori information. Furthermore, it appears in practice that the this use of a priori information gives rise to artefacts in the reconstructed magnetic resonance image.
An object of the invention is to provide a magnetic resonance imaging method in which artefacts are more effectively avoided in the magnetic resonance image reconstructed from the sub-sampled magnetic resonance signals.
This object is achieved by the magnetic resonance imaging method according to the invention wherein said a priori information is taken into account as a constraint in said optimization.
The invention is based on the insight that the known method produces artefacts in the form of ugly cut-line effects and that these are effects due to the blunt setting to zero of pixel values without taking into account the unfolding of sub-sampled magnetic resonance signals in the decomposition process. According to the invention the a priori information is taken into account in the optimization as a constraint which is implemented, for example, mathematically while using one or several Lagrange multipliers. This results in more gradual employment of the a priori information in the decomposition or unfolding of the image information in the sub-sampled magnetic resonance signals. A simple optimization procedure consists of a least-squares fit method that is quite reliable and easy to implement.
The a priori information, for example, includes accurate information on the position of the object to be examined with respect to the field-of-view of the magnetic resonance imaging system. Notably the position of the patient""s chest wall and its motion due to breathing are taken into account in the a priori information as to for which pixel-positions (almost) zero pixel values are expected. The a priori information may also include information relating to the manner of the acquisition of magnetic resonance signals and effective filter settings from which information on the expected pixel-values can already be derived. The a priori information be may made available in the form of pre-set pixel values for the magnetic resonance image to be reconstructed. The a priori information may also pertain to a local permissible noise level in the reconstructed magnetic resonance image. In another example the a priori information stipulates that pixel values cannot exceed a preset ceiling value.
It is noted that the present invention is advantageously used in conjunction with several different forms of sub-sampling. The time required for the acquisition of the magnetic resonance (MR) signals is reduced by employing sub-sampling of the MR-signals. Such sub-sampling involves a reduction in k-space of the number of sampled points that can be achieved in various ways. Notably, the MR signals are picked-up through signal channels pertaining to several receiver antennae, such as receiver coils that are preferably surface coils. Acquisition through several signal channels enables parallel acquisition of signals so as to achieve a further reduction of the signal acquisition time.
Owing to the sub-sampling, sampled data contain contributions from several positions in the object being imaged. The magnetic resonance image is reconstructed from the sub-sampled MR-signals while using a sensitivity profile associated with the signal channels. Notably, the sensitivity profile is, for example, the spatial sensitivity profile of the receiver antennae, such as receiver coils. Preferably, surface coils are employed as the receiver antennae. The reconstructed magnetic resonance image may be considered as being composed of a large number of spatial harmonic components which are associated with brightness/contrast variations at respective wavelengths. The resolution of the magnetic resonance image is determined by the smallest wavelength, that is by the highest wavenumber (k-value). The largest wavelength, i.e. the smallest wavenumber, involved, is the size of the field-of-view (FOV) of the magnetic resonance image. The resolution is determined by the ratio of the field-of-view to the number of samples. In the event that the SENSE technique is employed, said ratio is referred to as the SENSE-factor which indicates the degree of sub-sampling.
The sub-sampling may be achieved in that respective receiver antennae acquire MR signals such that their resolution in k-space is coarser than required for the size of the field-of-view of the magnetic resonance image. The smallest wavenumber sampled, i.e. the minimum step-size in k-space, is increased while the largest wavenumber sampled is maintained. Hence, the image resolution remains the same when applying sub-sampling, whereas the minimum k-space step increases, i.e. the field-of-view decreases. The sub-sampling may be achieved by reducing of the sample density in k-space, for example by skipping lines in the scanning of k-space, so that lines in k-space are scanned that are spaced apart further than required for the size of the field-of-view of the magnetic resonance image. The sub-sampling may be achieved by reducing the field-of-view while maintaining the largest k-value so that the number of sampled points is reduced accordingly. Owing to the reduced field-of-view sampled data contain contributions from several positions in the object being imaged.
Notably when receiver coil images are reconstructed from sub-sampled MR-signals from respective receiver coils such receiver coil images contain aliasing artefacts caused by the reduced field-of-view. On the basis of the receiver coil images and the sensitivity profiles the contributions in individual positions of the receiver coil images from different positions in the image are disentangled and the magnetic resonance image is reconstructed. This MR-imaging method is known as such under the acronym SENSE-method. This SENSE-method is discussed in more detail in the international application No. WO 99/54746-A1.
Alternatively, the sub-sampled MR-signals may be combined into combined MR-signals which provide sampling of k-space corresponding to the full field-of-view. In particular, according to the so-called SMASH-method sub-sampled MR-signals approximate low-order spherical harmonics which are combined according to the sensitivity profiles. The SMASH-method is known as such from the international application No. WO 98/21600. Sub-sampling may also be carried-out spatially. In that case the spatial resolution of the MR-signals is less than the resolution of the magnetic resonance image and MR-signals corresponding to full resolution of the magnetic resonance image are formed on the basis of the sensitivity profile. Spatial sub-sampling is achieved in particular in that MR-signals in separate signal channels, e.g. from individual receiver coils, form a combination of contributions from several portions of the object. Such portions are, for example, simultaneously excited slices. Often the MR-signals in each signal channel form linear combinations of contributions from several portions, e.g. slices. This linear combination involves the sensitivity profile associated with the signal channels, i.e. of the receiver coils. Thus, the MR-signals of the respective signal channels and the MR-signals of respective portions (slices) are related by a sensitivity matrix which represents weights of the contribution of several portions of the object in the respective signal channels due to the sensitivity profile. By inversion of the sensitivity matrix, MR-signals pertaining to respective portions of the object are derived. In particular MR-signals from respective slices are derived and magnetic resonance images of these slices are reconstructed.
The magnetic resonance imaging method of the invention is advantageously used for forming a diagnostic magnetic resonance image of a patient to be examined. This results in a high diagnostic quality of the magnetic resonance image which requires only a short acquisition time for picking-up the magnetic resonance signals. Hence, motion of and within the patient to be examined hardly disturbs the magnetic resonance image and rapid changes and dynamics such as a rapidly beating heart can be visualised by way of a rapid series of magnetic resonance images.
Furthermore, as more a priori information is employed in the reconstruction of the magnetic resonance image, a higher degree of sub-sampling can be allowed without substantial aliasing artefacts remaining in the magnetic resonance image. In particular the ratio of the sampling required for the field-of-view to the employed sub-sampling density may be higher than the number of independent surface coils or independent signal channels involved in the acquisition of sub-sampled magnetic resonance signals.
These and other aspects of the invention are further elaborated with reference to the preferred embodiments as defined in the dependent claims.
Preferably, the a priori information used in the reconstruction of the magnetic resonance image is obtained from an explorative magnetic resonance scan. Such an explorative magnetic resonance scan involves the acquisition of magnetic resonance signals prior to the acquisition of the sub-sampled magnetic resonance signals.
Preferably, the a priori information used in the reconstruction of the magnetic resonance image is obtained from an explorative magnetic resonance scan. Such an explorative magnetic resonance scan involves the acquisition of magnetic resonance signals prior to the acquisition of the sub-sampled magnetic resonance signals. The explorative magnetic resonance scan advantageously, acquires magnetic resonance signals that are encoded for positions in a three-dimensional volume. A priori information relating to specific cross-sections through that three-dimensional volume is then derived by multi-planar reformatting. The multi-planar reformatting is, for example, applied to the voxel values of the three-dimensional volume that is reconstructed from the explorative magnetic resonance scan. Hence, the number of required explorative magnetic resonance scans is low, that is, even a single explorative magnetic resonance scan may suffice.
The a priori information is advantageously obtained in various ways. For example, the a priori information can be derived from a previous examination of the same patient, from the setting of a preparatory sequence such as for setting presaturation slabs, from the setting of a spatial excitation profile, etc. In particular the magnetic resonance imaging method may include a scan of k-space where in same portions, such as the peripheral regions, k-space sub-sampling is applied while in other portions, such as a central region around k=0, full sampling of the magnetic resonance signals is applied. The a priori information is then preferably derived from the fully sampled portion of k-space.
The explorative magnetic resonance scan may also be employed to detect the actual spatial sensitivity profile. This is because the explorative magnetic resonance scan includes RF-excitation of spins in the object to be examined. Subsequently, the explorative magnetic resonance signals are generated, for example, as free induction decay signals or as echo signals. The spatial sensitivity profile is calculated from measurement of these explorative magnetic resonance signals. Thus, the same explorative magnetic resonance scan is employed to obtain both the sensitivity profile and the a priori information. Hence, the time required for preparations before the acquisition of the sub-sampled magnetic resonance signals remains quite short.
A magnetic resonance imaging system is often provided with a main receiver coil, such as a so-called quadrature body coil (QBC) which encompasses an examination space and picks-up signals from substantially the entire examination space. The main receiver coil usually encompasses a tunnel-shaped examination space. Furthermore, one or several surface coils are provided. The surface coils have a sensitivity range for magnetic resonance signals that is strongly spatially localized. The surface coils are placed on the patient""s body adjacent the region to be imaged.
According to the invention, the explorative magnetic resonance signals are picked-up by both the main receiver coil and by the respective surface coils. The explorative magnetic resonance image is reconstructed from the explorative magnetic resonance signals picked-up by the main receiver coil. Separate coil-images are reconstructed from the magnetic resonance signals from the separate surface coils. The sensitivity profiles are then obtained from the ratios of the brightness values in the coil images to the corresponding brightness values in the explorative magnetic resonance image. It has been found that a particular useful implementation involves interleaved signal acquisition with alternating acquisition of explorative magnetic resonance signals from the respective surface coils and acquisition of explorative magnetic resonance signals from the main coil.
Preferably, the explorative magnetic resonance image is reconstructed from extended magnetic resonance signals which are obtained by interpolation between the explorative magnetic resonance signals picked-up by the main receiver coil. In this implementation the explorative magnetic resonance signals may be acquired with the main receiver coil with a relatively low spatial resolution, i.e. with a low spatial density. This interpolation ensures that the spatial pixel density of the explorative magnetic resonance image equals the pixel density of the magnetic resonance image reconstructed from the sub-sampled magnetic resonance signals on the basis of the spatial sensitivity profiles. Hence, there is no need to for the spatial resolution of the main receiver coil and surface coils to be (approximately) equal. The a priori information taken into account is then represented by the explorative magnetic resonance image and the equal pixel densities enable the optimization to take into account all pixel positions in the reconstructed magnetic resonance image.
The invention also relates to a magnetic resonance imaging system. The magnetic resonance imaging system according to the invention is defined in the independent claim 7. The magnetic resonance imaging system of the invention is suitable for carrying out the magnetic resonance imaging method of the invention. This is achieved in practice by suitably programming a computer or micro-processor which controls the magnetic resonance imaging system.
The invention also relates to a computer program as defined in the independent claim 8. The computer program according to the invention enables the magnetic resonance imaging system to achieve the technical effects involved in performing the magnetic resonance imaging method of the invention. The computer program is loaded in the computer of micro-processor of the magnetic resonance imaging system. The computer program can be provided on a carrier such as a CD-ROM disk. The computer program may also be provided via a network, such as the world-wide web; the computer program can then be downloaded into the memory of the computer of the magnetic resonance imaging system.